Electro-Thermal Formation of Uniform Lying Helix Alignment in a Cholesteric Liquid Crystal Cell

Abstract

We demonstrated previously that the temperature of a sandwich-type liquid crystal cell with unignorable electrode resistivity could be electrically increased as a result of dielectric heating. In this study, we take advantage of such an electro-thermal effect and report on a unique electric-field approach to the formation of uniform lying helix (ULH) texture in a cholesteric liquid crystal (CLC) cell. The technique entails a hybrid voltage pulse at frequencies f1 and, subsequently, f2, which are higher and lower than the onset frequency for the induction of dielectric heating, respectively. When the cell is electrically sustained in the isotropic phase by the voltage pulse of V = 35 Vrms at f1 = 55 kHz or in the homeotropic state with the enhanced ionic effect at V = 30 Vrms and f1 = 55 kHz, our results indicate that switching of the voltage frequency from f1 to f2 enables the succeeding formation of well-aligned ULH during either the isotropic-to-CLC phase transition at f2 = 1 kHz or by the electrohydrodynamic effect at f2 = 30 Hz. For practical use, the aligning technique proposed for the first time in this study is more applicable than existing alternatives in that the obtained ULH is adoptable to CLCs with positive dielectric anisotropy in a simple cell geometry where complicated surface pretreatment is not required. Moreover, it is electrically switchable to other CLC textures such as Grandjean planar and focal conic states without the need of a temperature controller for the phase transition, the use of ion-rich LC materials, or mechanical shearing for textural transition.

Temperature‐Responsive Photonic Devices Based on Cholesteric Liquid Crystals

“…[80,81] Indirect heating by applying an electric field can also be used to generate photonic bandgap tuning in CLC systems in a fashion similar to that described earlier. [76,[82][83][84][85] Electrothermal color tuning is possible by applying an electric field between two conductive substrates [76,84,86] or using only one conductive substrate with a specifically designed interdigitated electrode pattern [85] Figure 5. Demonstration of a TR-RC CLC system (LC host: BL006, chiral dopant: ZLI-811, i.e., S811) based on the chiral dopant solubility.…”

“…Indirect heating by applying an electric field can also be used to generate photonic bandgap tuning in CLC systems in a fashion similar to that described earlier. [ 76,82–85 ] Electrothermal color tuning is possible by applying an electric field between two conductive substrates [ 76,84,86 ] or using only one conductive substrate with a specifically designed interdigitated electrode pattern [ 85 ] that generates an in‐plane alternating current (AC) electric field. To give an example of electrothermally driven bandgap shifting, a CLC system consisting of a nematic LC mixture MLC‐2138 and a chiral dopant S811 was used to fill a cell equipped with an interdigitated electrode pattern on top of a glass substrate.…”

Pseudo-dielectric relaxation-enabled electrothermal tuning of reflective color in a soft chiral photonic crystal

“…Besides, experiments using cells with various sets of cell parameters have also been carried out in our recent works. [26][27][28] It is concluded that the cell gap, the electrode area and the intrinsic dielectric anisotropy in LC used are responsible for the measured dielectric spectrum, thereby determining the profile of the pseudo-dielectric relaxation and the strength of the dielectric heating effect. For example, according to Eqs.…”

Polymer Stabilization of Uniform Lying Helix Texture in a Bimesogen-Doped Cholesteric Liquid Crystal for Frequency-Modulated Electro-Optic Responses

“…The uniformity of ULH formed with this electric field approach has been improved by mechanically shearing the cell substrates simultaneously [10,11] or by modifying the surface alignment conditions with alternate stacking of planar and homeotropic alignment [12], or with helical-pitch-matched cholesteric alignment layer [13]. Other electric field approaches-including those based on voltage-induced electrohydrodynamic instability [14,15], flexoelectric effect [16], textural transition [17], electro-thermal effect [18], and adoption of tri-electrode configurations [19,20]-have been successively proposed for generating ULH alignment in a planar-aligned cell. Nevertheless, most ULH textures as prepared by the above-mentioned voltage pretreatments would be irreversibly destroyed or even transferred back to the most stable Grandjean planar texture after the CLC helix is partially or completely unwound by external voltages or the phase transition often occurs unwantedly by thermal variation; hence, making them incompatible for utilization of the flexoelectric or dielectric switching in extensive electro-optic applications.…”